This invention relates to the field of additive metallization of substrates.
Many electronic applications require patterned metallization of nonconductive substrates for interconnection among electronic devices. Examples of such applications include high density packaging (multi-chip modules), antennas, flex circuits, printed wiring boards, and flat panel displays. The metallic interconnects are conventionally formed by subtractive processes. Modern additive processes attempt to overcome the drawbacks of subtractive processes.
In a subtractive metallization process, a surface of the substrate is first fully coated. Alternatively, a metal sheet can be laminated onto a flat substrate using adhesive. Selected portions of the metal plating are then etched to leave the desired patterned metallization. Vacuum-assisted physical vapor deposition and sputtering deposition are often used to achieve en masse plating. Physical vapor and sputtering deposition require high vacuum, and consequentially high capital equipment and operating costs. Those deposition processes can also result in poor adhesion of the metal to the substrate.
Full coverage metal plating can also be achieved by sensitizing a surface of the substrate with a palladium chloride/tin chloride bath and chemically reducing palladium ions to form catalytic clusters. Electroless plating followed by electrolytic plating deposits metal on the surface. This process can be costly due to the large number of wet processing steps, and the chemical dissimilarity between the metal coating and the substrate discourages chemical bonding therebetween. Consequently, the metal only weakly adheres to the substrate.
After full coverage metal plating, a layer of resist (a photoresist is often used) is deposited in a pattern on top of the metal layer, with the pattern corresponding to the desired metallization pattern. A subsequent etching step removes all the metal except that protected by the patterned resist layer. The etching process is usually time consuming and costly, and can require the use of materials unfriendly to the environment.
Additive processes have been proposed to overcome the environmental drawbacks of subtractive processes. The additive processes proposed achieve only thin layers of metallization, limiting their practical applications. See, e.g., Tokas et al., U.S. Pat. No. 5,348,574. The proposed processes also suffer from poor adhesion, just as with the subtractive processes. Some of the additive processes retain resist and etching steps; consequently, they suffer from the same environmental hazards as the subtractive processes. See, e.g., Hirsch et al., U.S. Pat. No. 5,192,581. Others have process limitations that limit their uses with widely available substrate materials. See, e.g., Orlowski et al., U.S. Pat. No. 5,153,023. Metal/foil adhesive processes use costly thick metal foils and poorly adhering low molecular weight polymer adhesives.
Accordingly, there remains a need for improved additive metallization processes, specifically for processes that provide improved adhesion between the substrate and the metallization and that can provide the increased metallization thickness required in practical applications such as printed wiring boards.
The present invention provides an improved additive process for metallization of substrates. The improved process comprises applying a catalyst solution onto the substrate. The catalyst solution can coat an entire surface of the substrate or can be selectively applied to only a portion of a substrate surface. The concentration of solvent in the layer of catalyst solution on the substrate surface can be reduced by heating the coated substrate. Metallic clusters can be formed in the remaining catalyst layer by further heating the substrate. Electroless plating can then deposit metal onto the coated portion of the substrate. Additional metallization thickness can be obtained by electrolytically plating the substrate after the electroless plating step. The improved process requires only one wet processing step (applying the catalyst solution onto the substrate) and does not require a high vacuum, so capital and operational costs are less than with existing processes. The improved process also does not require an etching step, avoiding the environmental hazards of chemical etching. The improved process results in improved adhesion of the metallization to the substrate and increased metallization thickness, making it suitable for a wider range of applications than existing processes.
Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.